The present invention relates to a filtration media and device comprising at least a layer having a structured surface that defines highly ordered fluid pathways.
An important segment of filtration media and filtering device development for removing particles from a fluid stream has been in the nonwoven fiber technology area. From the use of webs derived from meltblown microfibers to that of microdenier staple fibers, the trend has been to decrease fiber size in order to increase available surface area per unit volume of web. These nonwovens are generally polymeric based, entanglement bonded, low density webs that incorporate micron or near micron size fibers.
The principle mechanisms that control particle removal from a fluid stream by a fibrous filter are direct interception, inertial impaction, diffusion, and electrostatic attraction. Particle collection by interception occurs when a particle following a gas streamline strikes and is captured by the filtering surface. Inertial impaction results when particles deviate from the fluid stream to strike the fibers. Impacted particles in both cases adhere to the fibers by forces such as Van der Waals"" forces. Diffusional collection occurs when the Brownian motion of very small particles enhances the probability of their contact with the filtering surface. This motion causes the particles to deviate from fluid stream lines and collect on the individual filter fibers. Electrostatic collection is an important mechanism whereby charged particles are attracted to oppositely charged collection surfaces by coulombic attraction.
Fibrous fluid filters, especially gas filters, typically combine all four capture mechanisms. Nonwoven filters incorporate the advantages of these fibrous filters due to their inherent properties. However, limitations with nonwovens as filtration media also stem from their inherent properties. Nonwoven webs by definition are randomly formed structures that have limited geometric order. Limited order is caused by the variability between individual fibers and the degree of fiber to fiber conformation within the web. This limited order is manifested by gross irregularities caused by the formation of macrostructures known as shingles and fiber nests. Web macrostructures have local concentrations of fibers that cause pore size variability as well as mass variability across the webs. As a result, relatively large openings between the fibers allow particles through that should have been excluded, and small openings fill and become ineffective. In filter media design these limitations are moderated by the use of additional material at the cost of higher flow resistance across the filter. These effects can be compounded during use in filtration applications by the force of the applied fluid, which can alter the web structure and thus the efficacy of the filtration device. In addition, pressure loading of the web, wherein the web is mechanically formed into product, for example, a pleated structure, can also cause additional deformation of the fibers and web, resulting in a decrease in filtration efficacy.
Other limitations of high surface area nonwoven webs as filtration media occur when the filter employs thin flat layers of nonwoven web, such as in respirators, or the filter employs pleated layers in a more three-dimensional arrangement, such as in room, furnace or computer filters. Because of their respective usages, the fluid velocity across the face of the respirator type filters tends to be lower, whereas the fluid velocity across the face of circulating air filters, i.e. the room, furnace or computer filters, tends to be higher. In both situations, however, the nonwoven web material typically performs as a surface loading filter, thereby eventually resulting in surface blinding. In surface binding, the first encountered layers of filter material fill and clog with particulate matter removed from the fluid stream. Therefore, the filters are not effectively using the greater portion of the filter mass, and thus the filters"" performance is limited based on filter surface area rather than filter volume.
The use of multiple layers to increase filter efficiency, especially in respirator type filters, can cause an increase in flow resistance across the media as the fluid passes through the filter layers. Flow resistance is a function of the gas face velocity and the relationship of the size, orientation, and number of torturous channels through the filter. Generally, a filter media with more uniformly distributed surface area will achieve greater overall filtration efficiency permitting the use of less material and, in turn, reduce flow resistance across the media.
Flow resistance across a filter media is a general design constraint for any filtration device. Flow resistance is particularly problematic in lower face velocity applications because the fluid velocity is low even before filtering, and any resistance to flow within the filter will have a dramatic effect on its output. This flow resistance can cause problems with the overall fluid handling system in which the filter is used.
Pleated structures of smaller fiber nonwoven webs are often used in the higher face velocity applications to reduce flow resistance and improve service life. This is because there is more filtering surface in a given volume, thereby increasing the percentage of surface openings per frame area of filter. When the nonwoven web is composed of microfibers, however, pleated structures can sometimes reduce web loft, (see U.S. Pat. No. 5,656,368 to Braun et al.) and may be limited by the size of the microfibers used because smaller fibers are more likely to cause surface blinding. Larger fibers may cause the filter to suffer from reduced overall filtering capacity due to a decrease in the actual fiber surface area.
Another means of improving filter efficiency is through treatment of the filter fibers to make them more attractive to the particles or the like to be removed from a fluid stream. Treatment methods include both passive and active electrostatic charging of the fibers, application of tacky material to the fibers, application of chemical additives such as catalysts or other reactive agents, as well as application of other types of additives, including deodorizers, drying agents, disinfectants, fragrances, and ozone removing agents. Although treatment methods can enhance particle capture by the fibers, the filters are still subject to the deficiencies associated with random media, such as surface blinding and the flow resistance limitations discussed above. Examples of treated filter media include commercial filter products known as electrets, such as those available from 3M Company under the trade designation xe2x80x9cFiltretexe2x80x9d.
Other types of filter media available for particle removal from a fluid stream include woven and knit materials. These types of materials tend to have a more ordered structure, thereby making them less susceptible to the limitations inherent in nonwovens. These materials, however, have their own problems with controlling structures fidelity due to variability in constituent fiber material, fiber formation and web construction. In addition, other problems include limitations such as small enough pore formation, constituent material costs, and manufacturing costs.
The present invention overcomes the disadvantages and shortcomings of the prior art by providing a filtration media or filtration device that is efficient, is capable of depth-bed loading, functions at a low flow resistance, and has a high collection capacity. More specifically, the present invention provides a filtration media comprising at least a layer having a structured surface that defines highly ordered fluid pathways. Preferably, the filtration media of the present invention comprises a stack of layers having structured surfaces defining a highly ordered array of filter openings and fluid pathways through the filtration media.
The structured surfaces of the layers may comprise features defining channels that form the fluid pathways, or may comprise features such as discrete protuberances that form the fluid pathways. The filter openings defined by the stacked structured layers remove particles by exclusion. Non-exclusion removal of particles is facilitated by the surface area of the structured surface features.
Filtration media in accordance with the present invention has the advantage of being efficient and having a high capacity because it uses the full volume by performing as a depth-bed filter, instead of as a surface filter. It is easily and economically manufactured from a variety of materials, including inexpensive, flexible or rigid polymers. The structured surface features of the filtration media are highly controllable, predictable and ordered, and are formable with high reliability and repeatability using known microreplication or other techniques. The filtration media can be produced in a high variety of configurations to meet the filtration requirements of a given application. This variety is manifested in: structured surface feature possibilitiesxe2x80x94discrete channels, open channels, or protuberances; channel configurationsxe2x80x94wide, narrow, xe2x80x98Vxe2x80x99 shaped, and/or sub-channels; stack configurationsxe2x80x94bonded or unbonded, facing layers, non-facing layers, added layers, aligned channels, offset channels, and/or channel patterns; and filter openingsxe2x80x94pore size, pore configuration, or pore pattern. In addition, the layers may be treated for enhanced filtration or other purposes.
The aforementioned advantages are achieved by a filtration media formed from at least one polymeric layer having a structured surface defined within it. Layers may be configured as a stack with the structured surfaces of the layers defining a plurality of ordered inlets open through a face of the stack and corresponding ordered fluid pathways, or may be a single layer with a structured surface having a cap layer, or may be an uncapped layer having a structured surface. In a stacked or capped arrangement, the layers thus form an ordered, porous volume. The ordered fluid pathways of the filtration media may be defined by a plurality of flow channels formed within the structured surfaces of the layers, or they may be defined by a plurality of discrete protuberances formed within the structured surfaces of the layers.
The plurality of flow channels are preferably defined by a series of peaks, each having two sidewalls. The peaks may be separated by a planar floor or by sub-peaks forming sub-channels within the flow channels. The peaks may have heads that overhang adjacent flow channels. The flow channels of a layer having a structured surface may be all the same or may be different. Each layer of the filtration media may have the same flow channel configuration, or may be different. The flow channels on adjacent layers may be aligned or may be offset.
Pairs of layers of the filtration media may face one another, and facing layers may engage one another. Layers may have structured surfaces defined on both faces. Additional layers may be added to the stack. A cap layer may cover a portion of the top of the layer, and additional layers may be placed between adjacent layers of the stack. The layers of the stack, or a layer and a cap layer, may be bonded together. The layers may be formed from the same or different polymeric materials. The filtration media may be treated to enhance particle removal or to provide other benefits such as providing oil and water repellency, removing odors, removing organic matter, removing ozone, disinfecting, drying, and introducing fragrance. Treatment may include charging of the layers to form an electret, surface coating of the layers, or addition of treated layers.
The aforementioned advantages may also be achieved by a method of filtering using the filtration media of the present invention. This method includes providing the filtration media, positioning the filtration media in a fluid flow path, passing a fluid through the filtration media, and removing particles from the fluid in the filtration media. This method may further comprise slicing a portion of a stack of layers having structured surfaces into a specific thickness to use as the filtration media, treating a portion of the layers to provide filtration benefits, and directing fluid flow to a specific destination by configuring the ordered fluid pathways within the filtration media.
In addition, these advantages may be achieved by a method of making and using the filtration media of the present invention. This method provides at least one layer having a structured surface defining highly ordered fluid pathways. The method may provide a plurality of layers having structured surfaces defining highly ordered fluid pathways by stacking the plurality of layers with the structured surfaces to define a plurality of ordered inlets open through a face of the stack and corresponding ordered fluid pathways and thereby forming an ordered, porous volume. The method also includes positioning the ordered, porous volume in a fluid flow path, passing fluid through the ordered, porous volume, and removing particles from the fluid in the ordered, porous volume. This method may also further comprise bonding of a portion of the layers, slicing a portion of the layers into a specific thickness, treating a portion of layers to provide filtration benefits, and directing fluid flow to a specific destination by configuring the ordered fluid pathways within the filtration media.